In the bowels of Iran's uranium conversion facility in Isfahan strands of black and red wire stretch from the concrete wall to giant white tanks full of a volatile uranium compound. It is by these slender cords that the international community hopes to hold Iran's atomic ambitions in check.

The wires pass through a brass seal that has been soldered and marked in such a way that any attempt to divert the fuel to making a bomb would be spotted by UN inspectors. It is a nuclear trigger the world hopes will never be pulled.

With global tensions rising over Iran's nuclear intentions, the doors of the Isfahan plant were opened last week to a small group of journalists from Europe and America in a rare bid for transparency by the embattled but determined government in Tehran.

Ten miles south-east of the tiled mosques of Isfahan, Persia's old capital, the conversion plant is a cluster of squat yellow-brick buildings at the foot of some weathered sandstone crags, and ringed by anti-aircraft batteries dug into the surrounding semi-desert.

Inside the plant, a dense network of shining vats, pipes and gauges perform the alchemy of turning processed uranium ore, "yellow cake", into uranium hexafluoride (UF6), an elusive gas which is a halfway house to making both nuclear fuel and nuclear bombs.

Iranian officials insist they just want to make fuel, and presented the Isfahan plant as a display of scientific prowess and peaceful endeavour. "You'll be able to see for yourselves the purity of our work, and you'll be able to tell the outside world the good news," Hossein Simorg, the spokesman for Iran's nuclear industry, promised before a guided tour of the facility.

As far as the much of the outside world is concerned, however, Isfahan is a nuclear flashpoint. Almost exactly two years ago, the seals on the tanks of uranium hexafluoride were broken in front of inspectors from the UN's International Atomic Energy Agency (IAEA), in an act of defiance by the Iranian government - a blunt signal Tehran was not going to halt its nuclear progress in return for the incentives Europe was offering. The order was given for uranium conversion to resume at Isfahan after a two-year gap.

That decision, in August 2005, marked the start of a crisis that has been steadily worsening ever since, to the point where Washington is said to be studying potential bombing targets, the Iranian leadership is spouting apocalyptic rhetoric, and Europe - once more caught in between - is scrambling to salvage a peaceful solution.

For the time being, the wires are back in place at Isfahan, along with two IAEA cameras that stare down from high perches on either side. But the degree of international control is tenuous at best. The very fact that the machinery is humming at Isfahan puts Iran in contravention of UN security council resolutions, calling for all work related to uranium enrichment to be suspended.

The tour given to foreign journalists was a show of openness which backfired when the government changed its mind at the last minute over what it was prepared to show. But the trip was also meant to send a clear message: that Iran has no intention of giving up any part of its nuclear endeavour, which it regards as entirely within its rights.

The Iranian government has dug in deep, convincing its population that mastery of uranium fuel production is synonymous with development and prosperity.

Before the Isfahan tour, a promotion film was screened showing the production of the first UF6 at the plant in 2004. The Iranian government also claims to have mastered the next step in the process, the engineering feat involved in spinning the UF6 in a high-speed centrifuge and separating out a variant, or isotope, of uranium, that is highly fissile - uranium-235. The work is being done at a centrifuge plant being built in Natanz, to the northeast of Isfahan.

Spinning the UF6 gas until it is up to 5% rich in U-235 produces nuclear fuel. Keep spinning until it is 90% enriched and you have the makings of a bomb.

That - combined with the fact that Iran omitted to tell the IAEA about Natanz until its existence was revealed by an opposition group in 2002 - lie at the roots of the global scepticism over Iran's programme.

But there is another huge question mark hanging over Isfahan and Natanz: why is the government in such a rush to enrich fuel, when it has no nuclear power plants in which to use it?

There is a single reactor nearing completion at Bushehr, but it is being built under contract with Moscow, and it is only supposed to use nuclear fuel provided by Russia. However, the Russian government slowed down work on Bushehr this year over what it said was a payment dispute, but which was widely seen as a veiled effort to add to the pressure on Iran to halt enrichment.

The great sphere of the power plant hangs over the ancient Gulf port like a pale rising moon. Its heavy metal doors were also opened to journalists last week, and it was immediately apparent that the Russian go-slow has left a lot of work undone.

The Iranians are increasingly pessimistic about Bushehr's future. "They are playing with us. I'm sure they will not give us fuel for Bushehr. So we came to the conclusion we have to be self-sufficient," a senior government official said in Tehran. This is at the heart of Iran's justification for its programme. The outside world cannot be counted on, and Tehran believes that only once it has mastered uranium enrichment will it be taken seriously.

"We know they don't trust us, and we don't trust them," the official said.